Catching Panthalus isn't something you'll be able to do early on in Palworld. As with most other legendary pals, this newly-added giant whale is a tricky fight, and what's more, you'll have to complete a quest before you can even face it.
Despite the fact you get the Panthalus quest after the new tower boss added in 1.0, you can actually do it yourself at any time provided you visit the Ancient Civilization Researcher at the Deserted Islet. This means that if you're strong, but haven't progressed the towers much, you can still fight Panthalus and catch it.
